1. Field of the Invention
The present invention relates to a lens barrel having a built-in cam mechanism for moving a lens in an optical-axis direction.
2. Description of the Related Art
A lens barrel having a built-in cam mechanism is known. The cam mechanism is for moving a lens in an optical-axis direction. The lens is moved in association with zoom adjustment and focus adjustment by a motor drive or a rotational operation. The lens barrel comprises a cam barrel and a movable barrel. The cam barrel is formed with a cam groove, and the movable barrel has a cam follower for engaging with the cam groove. When the cam barrel is rotated around a barrel axis thereof, displacement of the cam groove is transmitted to the cam follower. Thereupon, the cam follower guided in an axis direction moves the movable barrel straightly so that the lens held by the movable barrel is moved along the optical axis.
In general, are provided plural pairs of the cam grooves and the cam followers so as to move the movable barrel smoothly and stably. When two pairs of the cam grooves and the cam followers are provided, for example, the cam follower is arranged at intervals of 180 degrees in a rotational direction of the cam barrel. When three pairs of the cam grooves and the cam followers are provided, for example, the cam follower is arranged at intervals of 120 degrees. The cam followers are arranged at even intervals on the identical circle. In case the lens barrel has two cam followers, the movable barrel is likely to fall down if the cam followers straightly arranged work as a rotary shaft. In view of this, it is preferable to provide three pairs of the cam grooves and the cam followers. Regarding prior art for preventing the movable barrel from falling down, is known a lens barrel in which there are cam grooves and cam followers arranged along the optical-axis direction in addition to the cam grooves and the cam followers arranged along the rotational direction of the cam barrel (see Japanese Patent Laid-Open Publication No. 4-343309, which is Japanese counterpart of U.S. Pat. No. 5,867,740).
In recent years, optical devices are improved so as to be more compact. Consequently, the lens barrel is downsized. With respect to the lens barrel having a small diameter, when a rotational amount of the cam barrel is large relative to a movement amount of the movable barrel, there arises a problem in that plural cam grooves partially overlap with each other. In the case three pairs of the cam grooves and the cam followers are provided or in the case the plural cam grooves are provided such as described in the above-noted Patent Publication, a width of the cam groove becomes narrow for the purpose of preventing the cam grooves from interfering with each other. The narrow cam groove impedes stable drive of the cam mechanism.